Breast cancer is a leading cause of death in women but extremely rare in men. A previous population-based cohort study found significant evidence for an elevated incidence of breast and ovarian cancers among female first degree relatives of men with breast cancer. Using an extension of logistic regressive models, we analyzed data on 842 first degree relatives and 131 wives of 153 male probands with breast cancer, to determine whether, after allowing for multifactorial familial correlations, single locus segregation could be the cause of this elevated incidence of female breast and ovarian cancers. We found evidence for a rare dominant Mendelian gene, perhaps BRCA2, segregating in these families and leading to early onset of breast and/or ovarian cancers.